The Indian scale and raga system is a great example of endless amounts of variations within a strict set of rules. It has, just like the Western system, 7 basic notes or "swaras" from which you can switch octaves, create flats and sharp notes, etc, but note where it differs:
As defined by Nazir Jairazbho from the ethnomusicology department at UCLA via wikipedia.com, "ragas are separated by scale, line of ascent and descent, transilience, emphasized notes and register, and intonation and ornaments."
This means it matters not only which notes you can use, but the order and stylization with which you use them. For instance, if you have the usual Sa, Re, Ga, Pa, Ne, Sa as the scale upwards, and Sa Ne Pa Ma Re Sa as the scale down, you cannot within a song really "alter" this order or the register of the notes (Eg: You can sing Sa Re Ga, Re Ga Pa, Ga Ga Pa Pa Ne Ne Ne Ne but you can NOT sing Sa Ga Pa, Ga Ne Sa- and the way you slide from one note to another also affects what raga a song is considered to be in.)
Each raga can even connote a mood, a time of day, a season, so on and so forth.
When the monsoon season comes, certain ragas are recommended. When the sun rises, you have yet another. When you are singing a lullaby, it is yet again a different raga.
Now, in my Western music class, as we listened to Bach, Brahms, and Wagner, we'd write papers on why they were able to achieve the effects that they did. "The oboe in the background makes you feel as if you are in a field of sheep. The sharp notes indicate aggression, the minor chord sadness." In Carnatic and Hindustani music, it's the other way around: people came up with these ragas with the effects in mind: we do not need to analyze a song, tempo, style, to understand it's meaning. We simply identify the appropriate raga. We are essentially given the end result of previous research, and told what ragas to use to achieve the particular effects we are looking for. Thousands of years of honing, beginning from the 8 basic "svaras" starting from the earliest Sanskrit slokas became a system with thousands of ragas and millions of categorized and systemized combinations.
When composing, ragas naturally fall into their expected roles. While working with composer Elizabeth Burke on The Rhythm Within, we'd find that the songs she had written - when we tried to add more classical Indian influences - would correspond appropriately with the recommended ragam. A song about change, the mood of fall, musing, ended up being perfect with the raga Kamaj...an early evening raga.
And guess where the complexity and brilliance of ragas come from? The earliest strains of language and the development Sanskrit.
"Sarvam sarvatmakam" I say. Everything is everything.